U.S. Published application 2005/0020926A1 discloses a scanning beam imager which is reproduced in FIG. 1 herein. This imager can be used in applications in which cameras have been used in the past. In particular it can be used in medical devices such as video endoscopes, laparoscopes, etc.
FIG. 1 shows a block diagram of one example of a scanned beam imager 102. An illuminator 104 creates a first beam of light 106. A scanner 108 deflects the first beam of light across a field-of-view (FOV) to produce a second scanned beam of light 110, shown in two positions 110a and 110b. The scanned beam of light 110 sequentially illuminates spots 112 in the FOV, shown as positions 112a and 112b, corresponding to beam positions 110a and 110b, respectively. While the beam 110 illuminates the spots 112, the illuminating light beam 110 is reflected, absorbed, scattered, refracted, or otherwise affected by the object or material in the FOV to produce scattered light energy. A portion of the scattered light energy 114, shown emanating from spot positions 112a and 112b as scattered energy rays 114a and 114b, respectively, travels to one or more detectors 116 that receive the light and produce electrical signals corresponding to the amount of light energy received. Image information is provided as an array of data, where each location in the array corresponds to a position in the scan pattern. The electrical signals drive an image processor 118 that builds up a digital image and transmits it for further processing, decoding, archiving, printing, display, or other treatment or use via interface 120.
Illuminator 104 may include multiple emitters such as, for instance, light emitting diodes (LEDs), lasers, thermal sources, arc sources, fluorescent sources, gas discharge sources, or other types of illuminators. In some embodiments, illuminator 104 comprises a red laser diode having a wavelength of approximately 635 to 670 nanometers (nm). In another embodiment, illuminator 104 comprises three lasers: a red diode laser, a green diode-pumped solid state (DPSS) laser, and a blue DPSS laser at approximately 635 nm, 532 nm, and 473 nm, respectively. Light source 104 may include, in the case of multiple emitters, beam combining optics to combine some or all of the emitters into a single beam. Light source 104 may also include beam-shaping optics such as one or more collimating lenses and/or apertures. Additionally, while the wavelengths described in the previous embodiments have been in the optically visible range, other wavelengths may be within the scope of the invention. Light beam 106, while illustrated as a single beam, may comprise a plurality of beams converging on a single scanner 108 or onto separate scanners 108.
In a resonant scanning beam imager (SBI), the scanning reflector or reflectors oscillate such that their angular deflection in time is approximately a sinusoid. One example of these scanners employs a MEMS scanner capable of deflection at a frequency near its natural mechanical resonant frequencies. This frequency is determined by the suspension stiffness, and the moment of inertia of the MEMS device incorporating the reflector and other factors such as temperature. This mechanical resonant frequency is referred to as the “fundamental frequency.” Motion can be sustained with little energy and the devices can be made robust when they are operated at or near the fundamental frequency. In one example, a MEMS scanner oscillates about two orthogonal scan axes. In another example, one axis is operated near resonance while the other is operated substantially off resonance. Such a case would include, for example, the non-resonant axis being driven to achieve a triangular, or a sawtooth angular deflection profile as is commonly utilized in cathode ray tube (CRT)-based video display devices. In such cases, there are additional demands on the driving circuit, as it must apply force throughout the scan excursion to enforce the desired angular deflection profile, as compared to the resonant scan where a small amount of force applied for a small part of the cycle may suffice to maintain its sinusoidal angular deflection profile.
As illustrated in FIG. 2, in one embodiment the scanner employs a concave objective lens or dome 212 having a partially reflective surface 214. The area of this surface will be appropriate for the medical application and the device design. In one embodiment it may be about 8 mm or less in diameter and in another embodiment it may be about 100-300 micron in diameter. This reflective surface may be integral to the dome 212, located centrally on the lens surface, as shown in FIG. 2 or it may be suspended or mounted on the incident side of the dome. The dome 212 has optical power and shapes the scanned beam 110 as it passes through the dome. In one embodiment, in order to view the areas directly behind the surface 214, the surface 214 is a material that is only partially reflective. The beam 208 emitted from the optical fiber 204 is directed to the reflector 214 via the shaping optic 210. The major portion (note FIG. 3) of the beam radiation is reflected by the reflector to the oscillating reflector 108 and from reflector 108 into the FOV as scanned beam 110. A smaller portion of the beam passes through the surface 214. This “leakage” radiation passing through the surface 14 is reflected from the field of view (FOV). This light is diffuse and when the reflector is not directing light/radiation through the surface 214, the light is not correlated with the point in the FOV that the scanner is interrogating. In this case it constitutes a source of noise that negatively impacts the SNR (signal to noise ratio). As shown in FIG. 1, a portion of the radiation reflected/scattered from the FOV, travels to one or more detectors 116 that receive the light and produce electrical signals corresponding to the amount of light energy received.
FIG. 3 is a graph of normalized detected beam intensity versus beam angle for an integral central reflective surface or “dot” reflector of the type illustrated in FIG. 2 having reflection coefficients of 0.75 and 0.95 respectively. FIG. 3 shows that less light is transmitted to and then returned to the detectors from the scene in the portion of the FOV covered by the projection of the reflective dot. For a reflective dot having a reflection coefficient of 0.75, 80% of the beam is reflected whereas for a reflective dot having a reflection coefficient of 0.95, less than 20% of the beam is reflected. Regardless of the reflection coefficient, as the angle at which the scanned beam is defected from the scanning reflector increases, e.g., at beam angles greater than about 25°, the beam intensity decreases, thereby also reducing the SNR of these wider angle beams.